CH637995A5 - Method for controlling and/or regulating a copper-plating bath operating without an external power supply - Google Patents

Description

The present invention relates to a method for automatically controlling and / or regulating a bath which deposits copper without an external power supply by measuring the mixed potential. The term "mixed potential" is to be understood as the oxidation / reduction potential which results from the transition of the copper ions into copper metal and the oxidation of a suitable reducing agent, e.g. Formaldehyde results (Milan Paunovic, PLA-TING, 68, page 1165, 1968).

In general there is no current, i.e. without external power supply copper-separating bath from the following components: a copper salt; a reducing agent which emits electrons on surfaces which are catalytically active for the oxidation thereof, in order to reduce and precipitate copper ions on them to metallic copper; one or more complexing agents which keep the copper in solution and prevent it from forming poorly soluble salts or from being reduced in the solution; and other components which serve, for example, to accelerate the metal deposition and to influence the crystal structure of the deposited copper layer and to stabilize the bath. A typical electroless copper plating bath consists of a copper salt, a complexing agent, a base, a component to improve the ductility of the deposited copper layer, a stabilizer and a wetting agent.

The composition can be, for example, as follows:

(1) about 0.02 to 0.08 mol / 1 CuS04 or CuCk;

(2) so much NaOH or KOH that the pH of the solution is between 11 and 14;

(3) about 0.01 to 3.0 moles of a reducing agent such as formaldehyde, or a hydride such as borohydride or amino borane;

(4) about 0.02 to 0.4 mole of complexing agents such as Na4EDTA, Rochelle salt, Quadrol, a polyalkanolamine or an amino acid;

(5) about 10 "3 to 10_l g / 1 ductility promoter such as NaCN;

(6) about 10 "9 to 10-3 g / 1 of a stabilizer, as well

(7) up to 5 g / 1 of a wetting agent such as alkylphenoxy or polyethoxyphosphate ester.

In addition to the components mentioned above, for example, from which the bath is originally composed, the copper deposition bath generally also contains a number of by-products during operation which inevitably form during the bath work.

The bath working conditions are appropriately kept as constant as possible by regular analysis of the bath components and appropriate chemical additives. The deposition bath usually has various parameters that can be measured and controlled, for example pH, copper ion concentration, reducing agent concentration, concentration of the complexing agent, temperature

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temperature, deposition rate, bath loading, type and intensity of the bath movement, rate of removal of the hydrogen formed during the reduction, concentration of the by-products formed during the reduction and the amount of poisoning substances that are already active in traces and negatively influence the deposition rate and properties of the metal deposit .

It is possible to maintain the bath working conditions by means of chemical analysis and to keep the appropriate concentration ranges by means of appropriate additives, but the maintenance of optimal conditions in this way is very difficult, since the loading and other conditions change in practical operation.

That is why the automatic control of all bathroom parameters is extremely desirable. Corresponding control procedures have already been described. However, these methods and corresponding devices only allow the control of a limited number of bath parameters.

In addition to the control methods already described, the determination and use of the mixed potential according to the invention opens up a further control option for the bath work, which surprisingly allows it to be optimized.

The inventors carried out extensive studies which made it possible to use the mixed potential to control the bath conditions, in particular as a measurement variable for controlling the activity of the bath or the concentration of the bath-stabilizing additives. The control of certain bath stabilizers by chemical analysis is very difficult, if not practically impossible, because of their usually extremely low concentrations.

Attempts in the past to use the mixed potential to determine the stabilizer-dependent bath activity have so far generally failed because too many other components also influence the mixed potential. Minor changes in the mixed potential can be caused by a number of parameters; their influence on the mixed potential increases the influences caused by changes in the stabilizer concentration, which can lead to the bath work coming to a complete standstill or the bath decomposing spontaneously and copper on all surfaces, i.e. also in the unwanted areas , precipitates. In summary, it can be said that the known measurement of the mixed potential does not provide any useful information about the activity as a function of the stabilizer and is therefore unusable for controlling it.

A new method for controlling and / or regulating a currentless bath has now been developed, which enables reliable bath work and is able to reduce the spontaneous deterioration of the bath solution.

According to the invention, the mixed potential is measured by means of two electrodes: copper is deposited on one electrode, while the other serves as a comparison electrode. The mixing potential can now be kept constant in a certain range. This range is generally determined by various factors, for example the composition of the bath solution and the nature of the comparison electrode.

The method according to the invention is characterized in that by means of two electrodes, one of which is in bath liquid and serves as a deposition electrode and the other serves as a comparison electrode, the mixed potential that forms between the electrodes is measured and this measurement for controlling and / or regulating one or several bath parameters is used.

To determine the mixed potential, both electrodes can be located within the deposition bath. 5 However, if the bath works at high temperatures, it is advisable to place the reference electrode in a separate chamber.

According to the method according to the invention, it is possible not only to measure the mixing potential, but also to simultaneously determine and / or regulate the factors which primarily influence the mixing potential, such as pH, activity of the stabilizer, temperature, activity of the Reducing agent and the copper concentration.

The temperature of an electroless bath can be determined and regulated using conventional methods. The commercially available glass electrodes can be used to measure the pH. The pH meter, which must be suitable for high pH values, can either be a combination electrode or a pH electrode in connection with a comparison electrode. A whole number of comparison electrodes can be used for this purpose, such as, for example, calomel electrodes or silver / chlorine-silver electrodes. A silver / chlorine-silver electrode can be used if two prerequisites are met: (a) there must be no liquid exchange via the connection, e.g. a microporous membrane, between the active bath solution and the comparison electrode solution, so that no silver ions get from it into the deposition solution, where they would lead to a spontaneous breakdown at the moment and in a very small amount; (b) in the area of a microporous membrane, the bath activity must be below normal, otherwise metal would deposit on the membrane and render it inoperable, which can be done by reducing the bath working temperature in this area or by interposing a salt bridge or by both at the same time can reach.

The temperature in the area of the reference electrode can be reduced simply by placing it in a cold bath solution outside the circulation area.

The Cu ++ ion concentration can be determined colorimetrically by measuring the absorption in the red spectral range. It should be noted that this measurement can be influenced by pH, temperature and some complexing agents; Corresponding corrections must therefore be made.

The cyanide concentration can be determined using commercially available special electrodes, provided that the bath activity is reduced to such an extent that no metal is deposited on the measuring or comparison electrode. Temperature and pH must also be taken into account for this measurement.

The formaldehyde concentration can also be determined colouri-55 metrically or photometrically by adding a reagent which gives a color reaction with formaldehyde. Another method is to lower the pH of the solution to such an extent that formaldehyde reacts with an excess of sodium sulfite added. The increase in the pH value caused by the reaction is a measure of the formaldehyde concentration, the starting pH being chosen so that the pH value arising after the reaction is in a favorable measuring range.

The concentration of the reaction by-products can be determined by determining the density of the bath solution.

The mixing potential of the bath liquid is basically the sum of the half reactions, which essentially result from the

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Oxidation of the reducing agent and the reduction of copper ions to metallic copper exist. If several oxidation / reduction processes run simultaneously on the same conductive surface, each reaction has its own balance and its own electrode potential. However, since there can only be one potential at a time on a conductive electrode surface, a constant potential, the mixed potential, forms during the course of the reaction.

The mixed potential of an electroless metal depositing bath is measured by means of two electrodes, the metal deposition electrode being in the working bath liquid and a comparison electrode generally in its immediate vicinity. The mixed potential results from the deposition reaction and is measured between the deposition and comparison electrodes.

The deposition electrode consists of a metal, for example platinum base with a copper coating, on which copper is continuously separated from the bath liquid. Almost every metal electrode is immediately covered with a copper layer after being placed in an electroless copper depositing bath.

Either a calomel or a silver / chlorosilver electrode can be used as the comparison electrode.

The determination of the mixed potential in the active bath liquid generally has certain difficulties. For example, the temperature of the working bath is between 40 and 95 ° C. If the comparison electrode is attached next to the deposition electrode under these conditions, copper will generally also deposit on the comparison electrode and thereby render it unusable; If the comparison electrode is placed further away from the deposition electrode, the measurement conditions are unstable. Satisfactory results can be achieved if the comparative electrode is placed in a separate chamber at reduced temperature and bath movement, which chamber is connected to the active bath liquid and the deposition electrode attached therein, the liquid bridge being kept as short as possible (5 to 10 cm) should be.

The chamber with the comparison electrode can be thermally insulated against that of the deposition electrode. The contact between the comparison electrode and the active bath liquid is preferably established via a connection which contains an inert saline solution and which is connected on the one hand to the chamber of the comparison electrode and on the other hand to the active bath liquid through a porous membrane. The activity of the bath liquid in the reference electrode chamber is generally reduced by reducing the bath temperature and movement.

There is generally no simple connection between the concentration of the individual bath components and the mixed potential; e.g. a change in the concentration of the bath components does not result in a linear increase in the mixing potential. If, for example, the concentration of formaldehyde is increased to such an extent that there are no longer enough copper ions available to increase the reaction rate, the mixing potential does not increase any further.

For the present invention, the mixed potential of the bath liquid should be kept within certain limit values which depend on the bath components and all other variable parameters. For commercial copper plating baths, the range between -200 mV and -1.5 V, measured against a standard calomel electrode, and -115 mV and 1.455 V, measured against a silver / chlorosilver electrode, was found to be the most favorable value. The mixed potential is advantageously set to between -600 to -850 mV, measured against a calomel electrode, and to -555 to -805 mV, measured against a silver / chlorine-silver electrode.

The best results are achieved at values between -630 to -760 mV (standard calomel electrode) and -585 to -715 mV (silver chlorosilver electrode).

When using a standard calomel electrode, the measured values deviate from other electrodes, for example silver-chlorine-silver electrodes, by about 45 mV.

In practice, a deviation of the mixed potential from the specified setpoint of ± 5 mV to ± 50 mV can be accepted; however, the value is preferably kept within limits between ± 10 to ± 25 mV. For example, the target value for a specific copper plating bath based on EDTA is -675 mV, ± 10 mV or ± 25 mV.

The mixed potential is the determining or controlling variable according to this invention. The other bath parameters are determined and controlled accordingly, with some bath parameters having a greater influence on the mixing potential than others. For example, a positive mixing potential, i.e. of more than -600 mV, can be reduced to the desired value in a simple manner by increasing the pH or the formaldehyde content of the bath liquid. If the mixed potential of the bath has slipped too far into the negative, it can be brought back to the setpoint by adding stabilizer solution.

The device that can be used for the method according to the invention determines the individual bath parameters and sets them to the desired values. After practically all parameters have a determining effect on the mixed potential, it is practical to determine and set only the bath parameters that have the greatest influence on the mixed potential. These include copper ion concentration, active concentration of the reducing agent, active concentration of the stabilizer, pH value and temperature. The device is preferably used to measure the mixing potential and to determine the temperature, pH, kufperion concentration and stabilizer concentration and to keep it within predetermined values. The device is preferably designed such that the corresponding additions are effected as a result of the measurement and the temperature is also set automatically. A preferred device measures the mixed potential and adjusts the temperature, copper ion concentration and pH and at the same time adjusts the active stabilizer and reducing agent concentration accordingly, so that the mixed potential remains within the predetermined range. In one embodiment, the mixed potential is measured and, accordingly, the activity of the reducing agent and / or the stabilizer is set, and the mixed potential is thus kept within the predetermined target values; In addition, the temperature is automatically determined and regulated and the copper ion concentration, pH and cyanide concentration are always kept at their target values.

The parameters influencing the mixed potential can be measured with the aid of the device described above, which is adapted to the bath parameters to be determined and / or to be controlled and which determines both each parameter individually and the mixed potential. A suitable device for determining and automatically controlling the temperature, copper ion concentration and pH value and for measuring the mixed potential is shown in FIG. 1.

Such a device consists of a pump 1 for pumping out a control amount of the bath liquid; this test liquid, that of the bath liquid in the separation tank 2

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is removed, is passed through a valve 3 into the chamber 4, where the deposition electrode 5 is located. Adjacent to the chamber 4 is the chamber 6 with the comparison electrode 7, which together with the deposition electrode 5 serves to measure the mixed potential. The chamber 4 contains a thermistor 8, which can also be arranged in the position 9 or on the degasser 10. The device 10 serves to degas the liquid before it reaches the measuring device. The test liquid is passed from the chamber 4 through the heat exchangers 11 and 12 and then into the chamber 13, which is used for the colorimetric determination of the copper ion content (14). The test liquid is passed from the chamber 13 into the chamber 15, which also contains a thermistor 16 and a combination of pH / comparison electrode 17.

Then the test liquid flows back via the first heat exchanger 11 into the deposition tank. The second heat exchanger is cooled with water from outside the system.

Another embodiment of the present invention is shown in FIG. 2 and is used to measure temperature, copper ion concentration, pH, cyanide and formaldehyde activity and to determine the amount of by-products that occur during bath work.

The test liquid is removed from the bath liquid 18 by means of a pump 19 and passed through the valve 20 into the chamber 21 which contains the deposition electrode 22 which, in conjunction with the comparison electrode 24 in the adjacent chamber 23, serves to measure the mixed potential. The chamber 21 can also contain a thermistor 25, which can also be attached in position 26 or on the carburetor 27.

The test liquid is passed from the chamber 21 via the heat exchangers 28 and 29 into the chamber 30, which contains the copper colorimeter 31 and the cyanide electrode 32. The liquid is then passed into the chamber 33, which contains a second thermistor 34 and a pH measuring electrode 35. A part of the test liquid is then passed into chamber 36, in which its density and thus the amount of by-products formed are determined with the device 37. The remaining test liquid is fed into the reagent pump 38 which pumps it into a device where it is loaded with air and sodium sulfite is added. From there the liquid reaches a mixing vessel 39 in which there is an acid. The supply of sodium sulfite and acid comes from the storage vessels 40 and 41. The liquid enriched with acid and sodium sulfite is led from the mixing vessel 39 into the chamber 42, where its pH value is determined with the aid of the pH comparison electrode 43. The increase in pH is proportional to the amount of formaldehyde present.

Typically, the test liquid is drained through pipe 44 as waste liquid. However, if its composition corresponds to that of the bath composition, it can also be returned directly from the chamber 36 to the separation tank 18 together with the other part of the test liquid.

The test liquid from chamber 36 is passed through the heat exchanger 28 and back into the separation tank 18; the second heat exchanger 29 is cooled with cooling water from a source located outside the system.

The devices shown in FIGS. 1 and 2 also contain devices for automatic registration and implementation of the signals supplied by the measuring devices. Each test device generates a millivolt signal, which is picked up by a type of potentiometer, such as a digital control potentiometer. The device receives this

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Signal, checks whether this is within the specified target values and makes the corresponding notification.

The device expediently contains devices for measuring and triggering corresponding reactions to the measured values. The device usually reacts as an electrical signal, preferably in millivolts. Each generated signal is e.g. caught by a kind of potentiometer, to which certain limit values are specified. The potentiometer in turn generates a signal when the collected signals are outside of these predetermined limits, which means e.g. the addition of a certain amount of a certain chemical can be effected. If the device detects that the pH is too low, it will add a certain amount of a certain alkaline solution. In another embodiment, the signal can trigger the addition of stabilizer liquid if the mixed potential shows a value that is too far in the negative range, or, in the opposite case, triggers the addition of formaldehyde as a reducing agent if the value of the mixed potential increases is far in the positive range.

In a further embodiment, in which the formaldehyde content, as shown in FIG. 2, is kept constant, deviations in the values of the mixed potential can have other causes. An indication of the addition of stabilizer is a negative tendency; If the signal for the mixed potential tends in the other direction, an accelerator or a formaldehyde oxidation catalyst must be added.

Comparative Experiment A The copper deposition bath used was composed as follows:

Copper sulfate

10 g / 1

Tetrakis (2-hydroxypropyl)

ethylenediamine

17 g / 1

Formaldehyde (37%)

15 ml / 1

pH

12.8

temperature

28 ° C

Sodium cyanide

25 mg / 1

Potassium sulfide

0.4 g / 1

2-mercaptobenzothiazole

0.04 g / 1

The copper ion content of this deposition bath was constantly checked by means of a colorimeter, which at the same time controlled the required addition of copper sulfate. The tetrakis (2-hydroxypropyl) ethylenediamine was not consumed during the coating, so that daily or weekly checks and, if necessary, additions were sufficient. A 37% formaldehyde solution was added in proportion to the carbon sulfate addition; In addition, the formaldehyde content was analyzed daily and appropriate amounts were added. The pH was constantly checked, using a glass electrode and a calom electrode as a reference electrode. The signal generated during the pH measurement simultaneously controlled the addition of NaOH. Sodium cyanide was added as an aqueous solution proportional to the NaOH. The stabilizer additions potassium sulfide and 2-mercaprobenzothiazole were measured according to the amount of copper plated and the elapsed time.

The mixing potential was determined between a plate in the deposition bath and the calomel electrode, which serves as a comparison electrode for the pH measurement. The mixed potential in this case showed very fluctuating values between -520 mV and -720 mV.

The deposition bath was difficult to control; either

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the copper was also deposited on the vessel walls and the bath tended to spontaneously decay; or, if enough sulfur-containing stabilizer was added, the deposited copper turned dark, became passive and the deposition very soon came to a complete standstill.

The following examples illustrate the invention.

Example I

The determination of the mixed potential after comparative experiment A was modified in such a way that a silver / chlorine-silver electrode was used as the comparative electrode and this was attached in the immediate vicinity of the deposition electrode. However, it had to be determined

that if the distance between the reference and the deposition electrodes was too small, copper was also deposited on it, so that it became unusable for the measurement. The comparative electrode was therefore placed in a separate chamber filled with cooled bath solution, which was connected to the active deposition bath and thus to the deposition electrode via an intermediate chamber filled with an inert salt solution. Satisfactory results have been achieved with this device. The electrodes were connected to a differential amplifier; the signal emitted by this was used to control the stabilizer addition. An aqueous solution of potassium sulfide and 2-mercaptobenzo-thiazole in a ratio of 10: 1 served as stabilizer. The setpoint for the mixed potential was -635 mV ± 10 mV. The deposition bath controlled in this way worked excellently; the copper deposited was shiny and ductile, and the bath did not tend to spontaneously decay, nor did the copper surface show any signs of passivation.

Example II

The bath of A was used in a tank that contained a circulation pump. One tank wall was lower than the other and served as an overflow. The pump pumped the overflow fluid back into the tank. This overflow device made it possible for oxygen to be absorbed and the hydrogen formed during the reaction to escape at the same time. The mixed potential was -600 mV; the deposited copper layer was dark and the deposition process stopped completely after a few hours.

By increasing the liquid layer in the overflow vessel, the uptake of oxygen was reduced and the release of hydrogen was made more difficult, as a result of which the mixing potential changed and now showed a value of -640 mV. The height of the liquid in the overflow was now around 3A of the height in the separation tank, whereas previously it had only been around 'A. By adjusting this height and by regulating the addition of sulfur-containing stabilizer, it was possible to keep the bath at a mixed potential of -635 mV ± 10 mV; it worked satisfactorily for two weeks without a break. This experiment shows that the mixed potential can be used to control and regulate the hydrogen and oxygen content.

Example III

An electroless copper plating bath was made according to the following composition:

Copper sulfate 12.5 g / 1 tetrakis (2-hydroxypropyl)

ethylenediamine 15.6 g / 1

Formaldehyde (37%) 3.5 ml / 1

Sodium hydroxide to adjust the pH to 12.95

Sodium cyanide 10.0 mg / 1

Potassium sulfide 1.0 mg / 1

2-mercaptobenzothiazole 0.1 mg / 1

GAFAC RE-610 0.14 g / 1

Temperature 53 ° C

Bath load 2.5 dmVLiter bath liquid

The copper content of this bath was constantly checked by colorimetric measurements and kept constant by appropriate automatically controlled additions. As already described, the pH was also continuously measured and, likewise automatically, the corresponding amounts of sodium hydroxide were added. Sodium cyanide was added in accordance with the sodium hydroxide additions, and formaldehyde in accordance with the amounts of copper sulfate required by colorimetric measurement.

The mixing potential was determined, as described in Example II, using a silver / chlorine-silver electrode as a reference electrode. As also described in Example II, the measured values were used for the automatic control of the stabilizer content; this consisted of potassium sulfide and 2-mercaptobenzothiazole in a ratio of 10: 1 in aqueous solution. The setpoint for the mixed potential was between -650 and -655 mV.

This type of bath works at elevated temperature, which means that the formaldehyde concentration must not exceed 5 to 6 ml / 1, otherwise spontaneous decomposition will occur. This means that all copper ions are reduced to copper at the same time, which can be easily recognized by the fact that the blue copper sulfate solution turns into a solution colored red by finely divided metallic copper, in which the finely divided red copper dust gradually sinks and the supernatant liquid is water bright. At the same time, there is violent hydrogen evolution, which causes the bath to foam up.

A strange phenomenon was observed in the bath described here: due to a switching error, the valve for the addition of the formaldehyde solution did not close and the concentration of the formaldehyde rose to 22 ml / l. Contrary to expectations, there was no spontaneous decay, but the bathroom continued to work normally. The explanation for this was that due to the falling value for the mixed potential, stabilizer was constantly added. This went well until the stabilizer supply was exhausted after 16 hours; then the bath decomposed within half an hour.

Example IV

The bath from test A was controlled automatically and the amount of copper deposited on a platinum electrode after 20 minutes was determined, giving a measure of the deposition rate. At the same time, the mixed potential was measured and it was found that it behaves exactly proportional to the deposition rate.

Example V

An electroless copper plating bath of the following composition was produced:

Copper sulfate 0.036 mol / 1

pH 12.9

Quadrai (former compound) 0.08 mol / 1

Temperature 53 ° C

Formaldehyde 0.05 mol / 1

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Sodium cyanide GAFAC RE-610

30 mg / 1 0.1 g / 1

The copper ion content, pH, temperature and formaldehyde concentration were constantly checked and, if necessary, the appropriate additions were made automatically. The cyanide ion concentration was measured and adjusted to -150 mV using a calomel and an orion cyanide ion electrode.

An aqueous solution of potassium polysulfide was used as the stabilizer and was added whenever the mixed potential measured with the aid of a silver / chlorine silver comparison electrode fell below -670 mV. A 37% formaldehyde solution was added whenever the mixed potential rose above -640 mV. The bath worked satisfactorily, the deposited copper was shiny, ductile and even and was only found in the desired places.

Continued analysis during the bath work showed that the formaldehyde concentration was kept between 0.01 and 0.04 mol / 1.

Example VI

To illustrate the invention in its effect on the adjustment of the formaldehyde content alone, a bath of the following composition was produced:

Copper sulfate pH

Quadrol formaldehyde GAFAC RE-610 temperature

Copper ion concentration, pH and temperature were constantly checked and the necessary additions were controlled automatically. Formaldehyde in 37% solution was always added when the mixed potential indicated a more positive value than -640 mV.

The bathroom worked well; the copper deposited was shiny and ductile; copper deposition on the tank walls was not observed.

Example VII

This example illustrates the use of the mixed potential to control the bathroom ventilation. A bath of the following composition was produced for this purpose:

Copper sulfate quadrol formaldehyde sodium hydroxide; Sodium cyanide temperature

0.04 mol / 1 0.06 mol / 1 0.26 mol / 1 0.36 mol / 1 0.0002 mol / 1 24 ° C

When air was blown through, the bath showed a mixed potential of -703 mV; if the air flow was interrupted, the potential slowly decreased to —720 mV.

Example VIII

An electroless bath in which Chel DM 41 is used as a complex and which has the following composition; tongue was produced:

Copper sulfate Chel DM 41 formaldehyde 20 sodium hydroxide temperature GAFAC RE-610

7.5 g / 1 22 ml / 1 15 ml / 1 6 g / 1 27 ° C 0.1 g / 1

0.036 mol / 1 12.9 0.08 mol / 1 0.05 mol / 1 0.1 g / 1 53 ° C

Initially, the mixed potential was -720 mV (measured 25 against a standard calomel electrode). The copper deposit was colored dark. Sodium cyanide was added until the potential was at -680 mV; the copper deposited at this value was shiny and ductile.

30 Example IX

The mixed potential can also be used to control and regulate the bath movement. If the bath is moved vigorously, it absorbs oxygen, the action of which increases the mixing potential; if the bath movement 35 is stopped, the mixed potential drops again. The bath used to illustrate this effect was composed as follows:

Copper sulfate 40 Rochelle salt sodium hydroxide formaldehyde temperature

14.6 g / 1 7.5 g / 1 7.5 g / 1 38 ml / 1 26 ° C

45 The mixing potential was initially —720 mV without bath movement. After switching on the bath movement device, the potential increased. The bath movement was now adjusted so that the mixed potential remained constant between -640 mV and -680 mV. The copper so deposited under these conditions was shiny and ductile.

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Claims (14)

637995 1. A method for automatically controlling and / or regulating a copper plating bath operating without an external power supply, characterized in that by means of two electrodes, one of which is in bath liquid and serves as a deposition electrode and the other serves as a comparison electrode which is located between the Mixed potential forming electrodes is measured and this measurement is used to control and / or regulate one or more bath parameters. 2. The method according to claim 1, characterized in that one or more of the following bath parameters are controlled by the mixing potential: copper ion concentration, active concentration of the reducing agent, active concentration of the stabilizer, active concentration of the cyanide ions, hydrogen ion concentration or pH value and ion concentration of the complexing agent . 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that one or more of the following bath parameters are controlled as a function of the mixed potential: bath temperature, bath movement and bath ventilation. 4. The method according to claim 1, characterized in that formaldehyde is used as the reducing agent of the copper depositing bath without external power supply, and that the bath work is regulated by the automatic control of the formaldehyde content as a function of the mixed potential. 5. The method according to claim 1, characterized in that a standard calomel or a silver / chlorine silver electrode is used to measure the mixed potential as a comparison electrode for the deposition electrode. 6. The method according to claim 5, characterized in that the two electrodes are coupled to one another via an intermediate vessel which is filled with a chemically inert substance or by means of a porous wall. 7. The method according to claim 6, characterized in that the intermediate wall is microporous and the bath activity in the vicinity of this intermediate wall is reduced so as to avoid copper deposition on this. 8. The method according to claim 5, characterized in that the temperature in the vicinity of the comparison electrode is lower than that in the deposition bath. 9. The method according to claim 1, characterized in that by checking one or more of the bath parameters, the mixed potential does not deviate by more than ± 5 mV to ± 50 mV from a nominal value lying between −200 mV and −1.5 V. 10. The method according to claim 9, characterized in that the mixed potential does not deviate from its target value by more than ± 10 mV. 11. The method according to claim 1, characterized in that the mixed potential is determined and that this information is converted into signals which are used to control one or more bath parameters so as to keep the mixed potential within predetermined limits. 12. The method according to claim 1, characterized in that the mixed potential is determined and the active concentration of the stabilizer and the reducing agent are adjusted so that the mixed potential is within predetermined limits, while the other bath components, preferably by automatic control and regulation, at their setpoints being held. 13. The method according to claim 1, characterized in that the mixing potential is determined and then, if this drops below a predetermined setpoint, as much stabilizer solution or reducing agent is added until the setpoint is reached again. 14. The method according to claim 13, characterized in that air or oxygen is added to the bath as a stabilizer or the air or oxygen addition is effected by bath movement, or is used for bath movement.